Image intensifier devices multiply the amount of incident light they receive and provide an increase in light output, which can be supplied either to a camera or directly to the eyes of a viewer. Image intensifiers are constructed for a variety of applications and hence vary in both shape and size, with proximity focused image intensifiers comprising a particular type of image intensifier having the smallest size and weight of all categories of image intensifiers. These devices are particularly useful for providing images from dark regions and have both industrial and military applications. For example, image intensifiers are used in night vision goggles for enhancing the night vision of aviators and other military personnel performing covert operations. They are employed in security cameras and in medical instruments to help alleviate conditions such as retinitis pigmentosis (night blindness). Such an image intensifier device is exemplified by U.S. Pat. No. 5,084,780 entitled TELESCOPIC SIGHT FOR DAY/NIGHT VIEWING by Earl N. Phillips issued on Jan. 28, 1992.
Image intensifiers include active elements, support elements and supply elements. The active elements include the photo-cathode (commonly called simply "cathode"), micro-channel plate (MCP), phosphor screen (screen), and getter. The cathode detects a light image and changes the light image into an electron image. The MCP amplifies the electron image and the screen changes the electron image back to an light image. The getter absorbs gas which is generated during operation of the tube.
The support elements comprise the mechanical elements which physically support the active elements of the tube. In a standard proximity focused tube these support elements are the vacuum envelope (known as the body), input faceplate (sometimes also called "cathode"), and the output faceplate or fiber-optic (also called "screen").
The supply elements in the tube include the chrome contact that is deposited on the faceplate to the cathode, the screen aluminum contact which is deposited on the fiber-optic or output faceplate, and the metalizing on the MCP glass. In addition the metal parts in the body assembly also provide electrical contact.
Finally there are packaging elements which perform other functions. The fiber-optics direct the image generated by the screen to a convenient position so that the system optics can properly direct the image to the ocular plane.
As is known, three major components of modem image intensifier tubes are the photocathode, phosphor screen (anode), and MCP disposed between the photocathode and anode. These three components are positioned within the evacuated housing or vacuum envelope, thereby permitting electrons to flow from the photocathode through the MCP and to the anode. In order for the image intensifier tube to operate, the photocathode and anode are normally coupled to an electric source whereby the anode is maintained at a higher positive potential than the photocathode. Similarly, the MCP is biased and operates to increase the density of the electron emission set forth by the photocathode. Furthermore, since the photocathode, MCP and anode are all held at different electrical potentials, all three components are electrically isolated from one another when retained within the vacuum housing.
Three major conceptions regarding the structure and function of image intensifier tubes exist in the prior art. The first prior art intensifier tube is an electrostatic inverting tube in which the image is inverted by an electron lens (FIG. 1). These tubes are often 25 mm format tubes constructed from the juxtaposition of conductive elements and dielectric elements. Prior art electro-static inverting tubes suffer numerous disadvantages because of their large size and weight, in addition to having numerous parts. In addition, these tubes suffer from distortion in the electron optics and, due to the electron optics, cannot use a Gen III photocathode. This problem is derived from the nature of the electron lens which requires a curved cathode plane. Gen III cathodes as a general rule must be flat. The large size results from the long length required to perform the electro-static image inversion. Moreover, these tubes were designed to have a fiber-optic input window which both increases the cost and weight of the device.
The second general type of prior art tube is a proximity focused tube (generally 25 mm) having a fiber-optic extender bonded to the tube as a partial solution to some of the electro-static inverter deficiencies (FIG. 2). The proximity focused tube is shorter than the electro-static inverter tube because the focusing of the electrons is accomplished by placing the imaging components close to one other. By doing so, however, the image is not inverted. Image inversion, which is required by most goggle systems, is usually accomplished by a fiber-optic inverter which is then bonded onto the tube. Present products have achieved only a minimal weight savings due to package requirements for retrofitting existing devices. While shorter lengths could be realized by making the fiber-optic inverter shorter, design problems including large tube diameters, with respect to the active diameter, and a large input fiber-optic faceplate as the electrostatic inverter tube still exists.
The third prior art tube comprises a tube (generally 18 mm) with the inverter inside the vacuum envelope or housing (FIG. 3). This tube replaces the fiber-optic faceplate with a glass faceplate, thereby providing higher resolution at lower cost, and makes the fiber-optic inverter part of the image tube envelope. While this tube has the smallest packing density of all the prior art tubes, several disadvantages still exist. First, the thickness of the faceplate makes an objective lens design more complex and costly. This was a trade-off in changing from a fiber-optic to glass faceplate. Moreover, this design is susceptible to stray light in the cathode faceplate.
In view of the prior art, there exists a need for an improved image intensifier tube having a housing including the inverter portion and that is both small and lightweight. Furthermore, the improved housing must maintain a reliable vacuum integrity, prevent stray light, and retain the MCP while electrically isolating the photocathode, MCP and anode from one another.